The major risks of allowing Internet access from an office environment are:
Risk of intrusion from outside into the office network using vulnerable PCs.
Unnecessary engagement of staff in Internet facilities such as social networks, blogs, chats resulting in loss of productivity.
Spread of viruses, malware through Internet and email services.
Possible exploitation of the computers by hackers to conduct cyber attacks, retrieve confidential information and cause network failure.
Though these risks are serious, we cannot totally block Internet in office. The following are some means of minimizing those risks:
Use of latest and robust software. Preferably licensed and legitimate software.
Regularly update and patch the systems.
Use good antivirus and anti-malware tools.
Do not allow access to social networks, blogs, YouTube from office. These kill time and reduce productivity the most.
Use proper security devices/software to block unwanted and malicious access / content from the Internet.
Make the staff aware about the risks and their mitigation techniques.